Minimizing analyte electrolysis in an electrospray emitter

The electrospray (ES) ion source is a controlled-current electrolytic flow cell. Electrolytic reactions in the ES emitter capillary are continually ongoing to sustain the production of charged droplets and ultimately gas-phase ions from this device. Under certain circumstances, the analytes under study may be directly involved in these electrolytic processes. It is demonstrated that a simple means to minimize analyte electrolysis is to exchange the normal metal emitter capillary of commercial ES sources with one made of fused silica. This change is shown to provide an ES mass spectrometric system of similar performance in terms of gas-phase ion signal generated for non-electroactive analytes and also assures minimal oxidation of electroactive analytes even at low (2.0 microl x min(-1)) solution flow-rates and high (millimolar) solution electrolyte concentrations.     

Redox buffering in an electrospray ion source using a copper capillary emitter.

An electrospray ion source used in electrospray mass spectrometry is a two-electrode, controlled-current electrochemical flow cell. Electrochemical reactions at the emitter electrode (oxidation and reduction in positive and negative ion modes respectively) provide the excess charge necessary for the quasi-continuous production of charged droplets and ultimately gas-phase ions with this device. We demonstrate here that a copper capillary emitter, in place of the more commonly used stainless-steel capillary emitter, can be utilized as a redox buffer in positive ion mode. Anodic corrosion of the copper capillary during normal operation liberates copper ions to solution and in so doing maintains the interfacial potential at this electrode near the equilibrium potential for the copper corrosion process [E degrees = 0.34 V versus standard hydrogen electrode (SHE)]. Fixing the interfacial potential at the emitter electrode provides control over the electrochemical reactions that take place at this electrode. It is shown that the oxidation of N-phenyl-1,4-phenylenediamine to N-phenyl-1,4-phenylenediimine (E(p/2) = 0.48 V versus SHE) can be completely avoided using the copper emitter, whereas this analyte is completely oxidized with a stainless-steel capillary emitter under the same conditions. Moreover, using N-phenyl-1,4-phenylenediimine, we demonstrate that reduction reactions can occur at the copper emitter electrode in positive ion mode. Emitter corrosion, in addition to redox buffering, provides a convenient means to introduce metal ions into solution for analytical use in electrospray mass spectrometry.     

Minimizing analyte electrolysis in electrospray ionization mass spectrometry using a redox buffer coated emitter electrode

An emitter electrode with an electroactive poly(pyrrole) (PPy) polymer film coating was constructed for use in electrospray ionization mass spectrometry (ESI‐MS). The PPy film acted as a surface‐attached redox buffer limiting the interfacial potential of the emitter electrode. While extensive oxidation of selected analytes (reserpine and amodiaquine) was observed in positive ion mode ESI using a bare metal (gold) emitter electrode, the oxidation was suppressed for these same analytes when using the PPy‐coated electrode. A semi‐quantitative relationship between the rate of oxidation observed and the interfacial potential of the emitter electrode was shown. The redox buffer capacity, and therefore the lifetime of the redox buffering effect, correlated with the oxidation potential of the analyte and with the magnitude of the film charge capacity. Online reduction of the PPy polymer layer using negative ion mode ESI between analyte injections was shown to successfully restore the redox buffering capacity of the polymer film to its initial state. Published in 2010 by John Wiley & Sons, Ltd.     

Insights into analyte electrolysis in an electrospray emitter from chronopotentiometry experiments and mass transport calculations

Insights into the electrolysis of analytes at the electrode surface of an electrospray (ES) emitter capillary are realized through an examination of the results from off-line chronopotentiometry experiments and from mass transport calculations for flow through tubular electrodes. The expected magnitudes and trends in the interfacial potential in an ES emitter under different solution conditions and current densities, using different metal electrodes, are revealed by the chronopotentiometry data. The mass transport calculations reveal the electrode area required for complete analyte electrolysis at a given volumetric flow rate. On the basis of these two pieces of information, the design of ES emitters that may maximize and those that may minimize analyte electrolysis during ES mass spectrometry are discussed.     

Electrochemical reduction of oxygen at a porous flow-through electrode

The electrochemical reduction of oxygen at a porous flow-through electrode is described with emphasis on the effects of concentration, flow speed and surface area. On a packed bed copper electrode in sulfuric acid, it was found that oxygen undergoes a two electron reduction process giving rise to H₂O₂.     

Sample pretreatment on a microchip with an integrated electrospray emitter

This study presents a microbead-packed PDMS microchip with an integrated electrospray emitter for sample pretreatment prior to sheathless ESI-MS. We prove the concept of analytical functions integrated onto a cm-sized area of a single bulk material. The microchip consists of two PDMS substrates replicated from SU-8 fabricated silicon wafer masters, bonded together after oxidation by corona discharge treatment. The channel within the microchip contains a grid structure that was used to trap 5 microm hypercross-linked polystyrene beads. The beads acted as a medium for sample desalting and enrichment. Electrical contact for the sheathless ESI process was achieved by coating the integrated emitter with conductive graphite powder after applying a thin layer of PDMS as glue. The coating as well as the bond of the PDMS structures showed excellent durability. A continuous spray was obtained from the microchip for over 800 h in a long-term electrospray stability experiment. Desalting and enrichment of neuropeptides from a physiological salt solution was successful by loading the sample onto the packed beads, followed by a washing and an eluting step. The results were obtained and evaluated using a TOF MS. An LOD of approximately 20 fmol (loaded onto the beads) for angiotensin II was obtained from a sample of neuropeptides dissolved in physiological salt solution.     

Declustering and fragmentation of protein ions from an electrospray ion source

A triple quadrupole mass spectrometer with a high pressure collision cell has been used to explore the declustering and fragmentation processes that may occur in the vacuum interface region of an electrospray or ionspray ion source. Using apomyoglobin as a model protein compound, collisional processes in Q2 were used to elucidate possible mechanisms which could occur in the orifice-skimmer region to affect the observed charge state distribution. The results indicate that charge loss or gain through collisional loss of a proton or electron does not occur; rather, higher collision energy results in better declustering of lower charge state ions, and fragmentation of higher charge state ions. The net result is an apparent shift toward lower charge state as the collision energy in the free jet region is increased. In addition, the data suggest that a mixture of heavily clustered monomers and possibly dimers and multimers are present in the expansion from ion source into vacuum, and it is this mixture which is acted on by the declustering field to produce the observed mass spectrum. The presence of these “superclusters” needs to be considered in any theory of ion desorption and transport processes in the source and interface region.     

On-line monitoring with an ion-selective electrode in a high-volume flow-through cell

A high-capacity poly(vinyl chloride) flow-through cell which can be used at a flow rate of 3000 ml min^-1 in a corrosive environment is described for the continuous on-line monitoring of copper ions in plant electrolytes. The copper(II) ion-selective electrode and double-junction reference electrode in the cell are coupled to a voltmeter and microprocessor-controlled instrumentation to achieve on-line monitoring. The system is designed to withstand the severe industrial environments and was field-tested in a copper refinery where continuous monitoring of copper is required. Cell design and results of the investigation on the determination of copper in plant electrolytes are described.     

Magnetic field focused ion accumulation for an internal bore liquid chromatography electrospray ionization Fourier transform ion cyclotron resonance mass spectrometer using a central trapping electrode

Presented is the application and evaluation of a magnetic field focusing central trapping electrode ion accumulation cell for a capillary liquid chromatography electrospray Fourier transform ion cyclotron (LC-ESI/FTICR) mass spectrometer. The ESI source and accumulation cell are located within the magnetic field to confine the radial motion of the ions, eliminating the need for elaborate focusing optics to transport the ions to the low-pressure analyzer cell for analysis. The central trapping electrode accumulation cell increases sensitivity by providing the necessary potential well in a confined volume to capture ions currently lost during the detection event of LC/FTICR experiments. With this electrode geometry the time needed to gate the ions into the analyzer cell is reduced and pump down delays are minimized. The decreased scan time improves LC resolution and increases the number of mass spectral scans per eluted component while maintaining appropriate base pressures for high performance ESI/FTICR. Results achieved with the central trapping electrode accumulation cell include an effective duty cycle increase from 10% to 40%, a S/N increase by a factor of 30, and a mass resolution increase of 80%.     

Electrolytic deposition of metals on to the high-voltage contact in an electrospray emitter: implications for gas-phase ion formation

The electrospray ion source is an electrolytic flow cell. Electrolytic reactions in the electrospray emitter maintain the production of charged droplets by this ion source that contain an excess of ions of one polarity. These redox reactions necessarily change the composition of the solution that initially enters the emitter. As a result, the ions ultimately observed in the gas phase by electrospray mass spectrometry (ESMS) may be substantially influenced by both the nature and extent of these electrochemical reactions. It is demonstrated in this paper that Ag(+), Cu(2+) and Hg(2+) ions in solution can be electrolytically reduced and deposited as the respective metals on to the surface of the high-voltage contact in the electrospray emitter in negative ion mode electrospray. The deposited metals are shown to be liberated from the surface by switching the electrospray high-voltage polarity to operate in the positive ion mode. The deposited metals are oxidized in positive ion mode, releasing the metal ions back into solution where they are detected in the electrospray mass spectrum. In a semi-quantitative analysis, it was found that up to 50% of the Ag(+) in a 2.5 microM solution was deposited on the high-voltage contact of the emitter as the solution flowed through the emitter. Deposition of Cu(2+) and Hg(2+) was less efficient. These data illustrate that in the analysis of metals by ESMS, one must be aware that both the concentration and form of the metals may be altered by electrochemical processes in the emitter. Hence reduction or oxidation of metals in the electrospray emitter, which may remove ions from solution, or change metal valence, would be expected to impact both quantitative metal determinations and metal speciation attempts using ESMS.     

Peculiarities of metal electrodeposition inside a flow-through porous electrode with low initial conductivity in oxidant containing solution

The earlier developed dynamic model of a flow-through electrode is used for studying how the variations in initial conductivity of a porous matrix κ_s,ini and a metal deposit affect the rate of metal deposition from an oxidant-containing solution for the direct-flow operation mode of the porous electrode. It is found that in contrast to an oxidant-free solution in which the decrease of κ_s,ini improves the uniformity of deposit distribution inside the porous cathode and increases the deposit final mass m _f, the opposite situation is observed in the presence of an oxidant, namely, a decrease in κ_s,ini, under otherwise similar conditions reduces the deposit mass and leads to its specific spatial distribution. The final metal deposit is divided into two separate fragments (rear and front) with a region of low conductivity of the initial porous matrix in between. Dynamics of the current and metal redistribution within the porous electrode, the reasons for the formation and stabilization of the rear fragment of coating, the correlation between the metal deposition rate and changes in the anodic zone position and intensity are discussed. It is shown that with the appearance of a specific profile of deposit distribution, the dependence of m _f on the metal conductivity develops a limit that differs considerably from the deposit final mass for an equipotential porous electrode.     

Characteristics of an amperometric flow-through detector with a renewable stationary mercury electrode

The construction of the detector is described and its performance is assessed for 1,4-benzoquinone by flow-injection analysis. The detection limit is 0.5 ng and the relative standard deviation of the peak current is 0.4% for a 102-ng sample. The peak current is highly sensitive to temperature changes of the mercury electrode system. The relative temperature coefficient is 15% K^-1, whereas the corresponding value with respect to the mobile phase is 0.6% K^-1. The dependence of the peak current on the volume flow rate is described and explained by appropriate models.     

A gated-beam electrospray ionization source with an external ion reservoir. A new tool for the characterization of biomolecules using electrospray ionization mass spectrometry.

In this work we describe a micro-electrospray ionization source equipped with an atmospheric pressure external ion shutter. The solenoid-activated shutter prevents the electrospray plume from entering the inlet capillary unless triggered to the 'open' position. When in the 'closed' position, a stable electrospray plume is maintained between the electrospray ionization (ESI) emitter and the electrically isolated face of the shutter. When the shutter is triggered, a 'slice' of ions is allowed to enter the inlet capillary and is subsequently accumulated in an external ion reservoir comprised of a radio frequency only (rf-only) hexapole and a pair of electrostatic elements. Following ion accumulation in the external ion reservoir, intact molecular ions of proteins, oligonucleotides, and noncovalent complexes can be stored for extended intervals (>30 minutes) prior to being transferred to the Fourier transform ion cyclotron resonance (FTICR) trapped ion cell for mass analysis. By introducing reactive gases directly into the external ion reservoir during the storage interval, ion-molecule reactions, such as H/D exchange, can be performed at high effective pressures. This scheme obviates the need for the long reaction times and delays associated with restoring base pressure in the trapped ion cell and allows H/D exchange reactions to be conducted in a fraction of the time required using conventional in-cell exchange approaches. The back face of the shutter arm contains an elastomeric material which can be positioned to seal the inlet to the mass spectrometer resulting in lower base pressure in the ion reservoir and the FTICR cell. Additionally, it is noted that blocking the ESI plume during non-accumulation events results in reduced fouling of the source electrodes and longer times between required source cleaning.     

Atmospheric pressure ion lens extends the stable operational region of an electrospray ion source for capillary electrophoresis-mass spectrometry

An atmospheric ion lens incorporated into an electrospray ion source for capillary electrophoresis-mass spectrometry (CE-MS) is found to extend the stable operational regions for both flow rates and electrospray ionization (ESI) voltages. The stable operating conditions for the ESI source with and without the ion lens were characterized. The results showed that the stable operation region was widest when the voltage difference between the sprayer and the ion lens ranges from 2.6 to 2.8 kV, and under these condition, the CE-MS interface can be adapted to a broader range of electroosmotic and modifier flow rates. Modeling of the electric field in the electrospray ion source with the ion lens suggests that the extension of the stable region is attributed to the flatter equipotential surfaces around the sprayer tip and higher electric field strengths in the rest of the interface region.     

Sheathless capillary electrophoresis-mass spectrometry using a pulsed electrospray ionization source

A sheathless interface has been developed for coupling CE with electrospray IT mass spectrometer. This interface utilized a pulsed ESI source. The use of a pulsed electrospray source allows the use of a sprayer with larger orifice, and thus alleviates the problem of column clogging during conductive coating and CE analysis. A pulsed ESI source operated at 20 Hz and 20% duty cycle was found to produce the optimal signals. For better signals, the maximum ion injection time in the IT mass spectrometer has to be set to a value close to the actual spraying time (10 ms). Using a sprayer with 50 microm od, more stable and enhanced signals were obtained in comparison with continuous CE-ESI-MS under the same flow rate (150 nL/min). The utility of this design is demonstrated with the analysis of synthetic drugs by CE-MS.     

Dynamics of the filling of a porous flow-through electrode,which is operating in a circulation mode,with metal

A model, developed previously for describing the filling of a porous flow-through electrode (PFE) with a metallic deposit, is used to demonstrate that, in contradistinction to a straight-through mode, a decrease in the concentration of metal in a circulating solution (approximately by an order of magnitude in the course of the time period required for filling a critical cross-section of PFE with metal) leads to a change both in the direction of the spatial redistribution of metal inside the porous matrix and in the dynamics of the variation of its basic parameters. In a straight-through mode, the metal distribution inside a PFE is defined by the action of two opposite factors (development of a surface inside the working layer L _ef at the expense of the growth of the diameter of fibers and, vice versa, its shrinking at the expense of a decrease in L _ef with time and the expulsion of the process of deposition of metal in the direction of the front end of PFE) and is characterized by nonmonotonous dependences of the current efficiency and the outlet concentration of metal on time. The predominant tendency in the case of a circulation mode is different: L _ef rises with time, which leads to a displacement of the process of the metal deposition in the direction of the rear end of PFE (at rear solution input), increase of maximal amount of deposit by approximately 1.4 times at L > L _ef, as well as to dependences of the current efficiency and the ratio between the concentrations of metal at the outlet and inlet of PFE that are monotonously decreasing with time. At the expense of a continuous variation of the effective working surface area of PFE and the mass transfer coefficient with time, the circulation mode is characterized by a nonlinear dependence of the logarithm of the metal concentration in the circulating solution on the electrolysis duration. A comparison of indicators that are characterizing the dynamics of the filling of a PFE with metal in the course of galvanostatic and potentiostatic modes of electrolysis is performed. It is established that the application of a potentiostatic mode of electrolysis under the conditions that provide for the predominance of the target reaction is accompanied by a slight decrease in the maximum quantity of metal, but a very significant decrease (by 5–6 times) in the specific spendings of electric energy.__________Translated from Elektrokhimiya, Vol. 41, No. 4, 2005, pp. 452–459.Original Russian Text Copyright © 2005 by Maslii, Poddubnyi, Medvedev.     

Development of a tandem time‐of‐flight mass spectrometer with an electrospray ionization ion source

A new tandem time‐of‐flight mass spectrometer with an electrospray ionization ion source ‘ESI‐TOF/quadTOF’ was designed and constructed to achieve the desired aim of structural elucidation via high‐energy collision‐induced dissociation (CID), and the simultaneous detection of all fragment ions. The instrument consists of an orthogonal acceleration‐type ESI ion source, a linear TOF mass spectrometer, a collision cell, a quadratic‐field ion mirror and a microchannel plate detector. High‐energy CID spectra of doubly protonated angiotensin II and bradykinin were obtained. Several fragment ions such as a‐, d‐, v‐ and w‐type ions, characteristic of high‐energy CID, were clearly observed in these spectra. These high‐energy CID fragment ions enabled confirmation of the complete sequence, including leucine–isoleucine determinations. It was demonstrated that high‐energy CID of multiply protonated peptides could be achieved in the ESI‐TOF/quadTOF. Copyright © 2010 John Wiley & Sons, Ltd.     

Automatic gain control in mass spectrometry using a jet disrupter electrode in an electrodynamic ion funnel

We report on the use of a jet disrupter electrode in an electrodynamic ion funnel as an electronic valve to regulate the intensity of the ion beam transmitted through the interface of a mass spectrometer in order to perform automatic gain control (AGC). The ion flux is determined by either directly detecting the ion current on the conductance limiting orifice of the ion funnel or using a short mass spectrometry acquisition. Based upon the ion flux intensity, the voltage of the jet disrupter is adjusted to alter the transmission efficiency of the ion funnel to provide a desired ion population to the mass analyzer. Ion beam regulation by an ion funnel is shown to provide control to within a few percent of a targeted ion intensity or abundance. The utility of ion funnel AGC was evaluated using a protein tryptic digest analyzed with liquid chromatography Fourier transform ion cyclotron resonance (LC-FTICR) mass spectrometry. The ion population in the ICR cell was accurately controlled to selected levels, which improved data quality and provided better mass measurement accuracy.